Crisis on Infinite Earths (Arrowverse)
Apparently it will be 5 episodes long, and they have a month of filming dedicated to it:
[url]https://tvline.com/2019/05/16/arrowverses-crisis-on-infinite-earths-crossover-schedule-legends/[/url]
[url]https://twitter.com/thedctvshow/status/1130110717074563072?s=21[/url]
(I don't know if the second source is reliable by the way).
I think this is the biggest crossover yet!
Wonder how many characters will be killed off, or not, but from what I know of the story it kinda has to be a bloodbath doesn't it?